So, depending on your skill bracket (which as you are newer, probably skews lower) this may happen more or less frequently, but I can only speak to when it happens to me.

When somebody abandons a game, it becomes safe to leave. This means, you can go to to the menu, leave the game, and immediately queue into a new game without any penalty. For myself, this is a fine solution because people don't abandon very often, but I can imagine this being a poor solution if abandons happen too frequently and you just want to play the dang game.

I also prefer just leaving a game with an abandon-er because the effect you describe is definitely real. There is an advantage for teams with 4 players, especially earlier in the game. In an ideal world the gold redistribution would be scaled to give both teams equal odds, but even that isn't perfect because your team technically still has the advantage if they can survive.

At some point in the game, the hero disadvantage should(?) outweigh the gold advantage. So, if you're looking for a general strategy for winning a game with a leaver on the opposing side, the goal is survival. You have to play as if the enemy team is way ahead in gold, because, well, they are. It also requires a much heavier emphasis on team play. Any one of their heroes is likely stronger than any one of your heroes, so your team will need to work together. There is also a double-edged sword to having fewer stronger heroes: it is more punishing for them if a teammate dies. So if and when you manage to kill an enemy hero, there may be bigger than normal openings for taking objectives (towers).